How to set a certain scene if object fall below a value?
You have a syntax error because you forgot the double point at the end of if statement.
You can change state of an actuator with current controller actívate or deactivate.

Your script should look like this:
import bge

# get the controller
controller = bge.logic.getCurrentController()

# get the game object that the controller is attached to.
obj = controller.owner

# get actuator attached to the controller named switchScene
act = controller.actuators[&quot;switchScene&quot;]

# Detects player's z position is equal or minor then zero
if obj.worldPosition.z &lt;= 0:
    # set scene name
    act.scene = &quot;Main&quot;
    # scene actuator mode: change to = 0
    act.mode = 0
    #change scene
    controller.activate(act)

Related : Python API controller.activate, Python API KX_SceneActuator
EDIT
Hi again.
About chicOrtiz comentary:
That error its because you dont have an actuator named &quot;switchScene&quot; conected to the current controller in the logic editor window (panel). as shown in the next image:

So i decide to make a blend example and found our next error.
We need to set actuator mode before set the scene. So the script should be:
import bge

# get the controller
controller = bge.logic.getCurrentController()

# get the game object that the controller is attached to.
obj = bge.logic.getCurrentScene().objects[&quot;Cube&quot;]

# get actuator attached to the controller named switchScene
act = controller.actuators[&quot;lascene&quot;]

# Detects player's z position is equal or minor then zero

if obj.worldPosition.z &lt;= 0:
    # set actuator mode : 
    # 0 = ?restart? ;   1 = ?restart?? ; 
    # 2 = set Scene ;   3 = set camera ; 
    # 4 = overlay   ;   5 = underlay
    act.mode = 2
    #set camera, it has to be in the same scene.
    act.camera = bge.logic.getCurrentScene().objects[&quot;Camera_main_scene.001&quot;]
    #Set actuator Scene   
    act.scene = &quot;Second_Scene&quot;
    #active actuator
    controller.activate(act)


Note: About if you need to use bpy or bge,
if you want a game always use bge
if you want animation movie, addon, etc use bpy
bpy and bge cant work together in anyway (in my understanding)
bpy only used inside blender (internal, cycles) dont bge